This invention relates generally to cardiovascular prostheses formed of synthetic fabrics. More particularly, it relates to such fabrics comprising multifilament synthetic yarns, to their manufacture, and to tubular vascular prostheses and trileaflet heart valve replacements formed of such fabrics.
The principal objects of the invention are to provide synthetic structures having a useful life of many flex cycles, to duplicate as closely as possible the mechanical properties of natural heart valves and blood vessels, and to avoid the introduction of blood clotting conditions.
The replacement of heart valves with prostheses has become a standard surgical technique. However, the prostheses currently in use do not entirely satisfy the above objects. Currently, most prosthetic heart valves rely for closure on the sealing of a ball or a flap against a gasket ring. With this construction the ball or flap is situated within the flow channel when lifted away from the gasket in the flow configuration. This is disadvantageous in two important respects. First, the pressure drop across the valve during the open or flow condition is greater than the pressure drop across the natural valve which causes a slight, but continuous and cumulative overload on the heart. Second, the presence of the ball or flap creates regions of turbulent flow tending to damage the red blood cells.
With the foregoing disadvantages in mind, research has been directed to developing leaflet valves more closely approximating the structure and functions of the human valve. The latter comprises then, flexible membranes that fold outward into the surrounding blood vessel in the open configuration, thus causing a minimum of disturbance to the flowing blood. In the closed configuration the leaflets form three contiguous pouches that are held in close and leak-proof contact by the pressure of the blood. As a result of the extreme lightness and flexibility of the leaflets the valve has a short response time, passing quickly from the fully closed to the fully open state, with the result that there is little energy loss in the flowing blood and a minimum of undesirable regurgitation.
These functional characteristics of the human valve result from the composite structure of the natural leaflet. This comprises an arrangement of bundles of collagen fibers embedded in a softer tissue material. The composite structure gives the leaflet good load bearing capacity, a high resistance to tear and sufficient softness and flexibility to make a good seal in the closed configuration. At the peak of the pressure pulse, the leaflet withstands a load exceeding 150 gm/cm along a line therein normal to the load.
Heart valve tissue is also anisotropic in its elastic properties, that is, the load-deformation characteristic in one direction is different from that in another direction. It has been found useful to define two particular directions for purposes of this description. These directions are parallel and perpendicular to the free edge of the leaflet, and correspond respectively to the circumferential and radial directions commonly referred to in the literature.
In the direction parallel to the free edge the natural leaflet extends very readily with increased load until an elongation of ten to twelve percent is reached at a load of one to two grams per centimeter of leaflet width. Upon further increase in load the resistance to further elongation increases greatly. In the direction perpendicular to the free edge the region of easy extension with increased load continues to approximately 20 percent elongation, at which the load is about 2 gm/cm. Upon further increases in load the resistance to further elongation, though greater than in the initial region, is not as high as it is in the parallel direction.
A recent development involves the use of stabilized pig heart valves as replacements for failing human valves. These valves embody some of the characteristics of human valves discussed above. However the collection, grading, sterilizing, fixing and storing of pig valves is complicated and costly. In consequence, a clear need has been realized for a trileaflet heart valve made entirely from synthetic materials.